Process for the disposal of sulfur

ABSTRACT

Process for the disposal of sulfur which comprises melting the sulfur, optionally in the presence of hydrogen sulfide, and injecting the molten sulfur into geological structures having a temperature ranging from 90 to 160° C.

This application is a National Stage application of PCT/EP04/07306,filed Jul. 5, 2004. Priority to Italian application MI2003 A 001470 isclaimed.

The present invention relates to a new process for the disposal ofsulfur.

More specifically, the present invention relates to a new process forthe disposal of sulfur coining from the purification treatment ofhydrocarbons of a fossil nature, for example crude oil or natural gas.

It is well known that sulfur can be present in considerable quantitiesin both extracted crude oil and natural gas. In this gas, the sulfur canbe present in the form of H₂S in a percentage molar quantity which canreach 10% and in particular certain cases it can even exceed 20%,referring to gas.

When present in high concentrations in gas, the hydrogen sulfide isseparated using various systems, of which the most widely used isabsorption in solutions of ethanol amines. Once it is in theconcentrated state, the hydrogen sulfide is transformed into elementalsulfur by means of the Claus process. At this point, it is necessary toallocate the sulfur which, for the last few years and potentially fortens of years to come, has a market characterized by an excessive offerwith respect to the demand. Sulfur is normally stored in elemental formas huge blocks which require continual monitoring and treatment of therun off water to avoid the acidification of the ground and surroundingunderground water. In addition to this, there is an increasingly strictlegislation on the part of states containing oil fields or natural gasreservoirs, which in some cases impose heavy penalties for the storageof recovered sulfur.

The Applicants have now found an innovative process for the eliminationof said large quantities of sulfur which envisages its injection intothe ground by pumping it in the liquid state into adequate geologicalstructures. These structures can consist of the field itself from whichthe crude oil or associated gas has been extracted or other adequategeological structures, such as abandoned mines or saline aquifers.

Thanks to the solution, object of the present invention, a permanentsulfur storage is obtained without any risk of its leaving the surfaceand contaminating usable groundwater or negatively interfering with theextraction process of crude oil/gas. Large accumulations of sulfur do infact exist in nature in indefinitely stable geological structures andwithout having a negative impact on the surrounding environment.

An object of the present invention therefore relates to a process forthe disposal of sulfur which comprises heating and melting the sulfur,optionally in the presence of hydrogen sulfide, and injecting the moltensulfur, in liquid and homogeneous form, into geological structureshaving a temperature ranging from 90 to 160° C.

The present invention envisages the pumping of sulfur in liquid formthrough surface pipes, well pipes and receiving geological structures.The molten elemental sulfur can come directly from the Claus process orfrom a surface storage site.

The pressure necessary for pumping the liquid obtained from theliquefaction of sulfur into a pipe can be calculated with the generalformula:ΔP=2f·ρ·u _(m) ² L/D _(eq)wherein L is the length of piping used for injection into the geologicalstructure. D_(eq) its equivalent diameter, u_(m) the average rate of thefluid pumped, ρ the density of the fluid pumped and f the frictionfactor which is a function of the roughness of the pipe and Reynoldsnumber:Re=D _(eq) ·u _(m)·ρ/μwherein μ is the kinematic viscosity of the fluid. The pumping devicecan be represented by a conventional pump capable of processing fluidsat temperatures ranging from 90° C. to 160° C. The viscosity values ofthe sulfur in the molten state within the temperature limits indicatedare specified in Table 1 below.

TABLE 1 Viscosity of molten sulfur in relation to the temperatureTemperature (° C.) μ (cPoise) 119 11.45 125 10.31 132 8.84 140 7.67 1506.62 157 6.72 160 11.6

The geological structures which are appropriate for receiving the moltensulfur are those having temperatures ranging from 90° C. to 160° C. and,preferably, those forming the reservoir itself from which the crude oilor natural gas containing sulfur in the form of organic compound orhydrogen sulfide, are extracted. Alternatively, geological structurescan be used, in a remote position with respect to the reservoir andhaving structural characteristics suitable for receiving and preservingthe molten sulfur.

For the application of the present invention, either matrix geologicalstructures or fractured structures, naturally or induced, can generallybe used. In all cases, the pressure and maximum injection flow-rate ofthe fluid forming the molten sulfur, optionally mixed with hydrogensulfide, can be determined by calculations and measurements well knownto experts in stimulation treatment of production wells or theproduction and running of water reinjection wells.

The present invention requires the injection of sulfur in liquid formwith a low viscosity. In the case of pure elemental sulfur, theappropriate conditions for the application of the present invention areobtained for temperatures ranging from 119° C. to 160° C. The field ofapplication of the disposal process of sulfur, object of the presentinvention, can be enlarged by mixing elemental sulfur with othercompounds capable of lowering the melting point of the mixture. A validand economical system for reducing the melting point of sulfur consistsin mixing it with H₂S under pressure. Table 2 below indicates themelting points (Tm) of sulfur in the presence of H₂S at differentpressure values. In this way it is possible to lower the melting pointof sulfur from 119° C. to 90° C. approximately.

TABLE 2 Influence of the H₂S pressure on the Tm of sulfur Melting pointof sulfur (° C.) H₂S pressure (bar) 119 0 115 1 111 12 106 24 100 39 10040 94 58 91 80 91 90 90 110 90 125 90 155

This lowering of the melting point of sulfur is verified, even if to alesser degree, also when hydrogen sulfide is mixed with other gaseswithout an analogous effect, such as, for example, methane or carbondioxide. From the following table it can be observed how, at a fixedpressure (for example 200 bar) the melting point of sulfur decreaseswith an increase in the concentration of H₂S in the mixture withmethane.

TABLE 3 Influence of the concentration of H₂S in methane on the meltingpoint of sulfur at 200 bar Melting point of sulfur (° C.) H₂S percentagein methane 119 0 117 4.5 115 9 114 14 109 25 92 100

The possibility of applying the present invention also using hydrogensulfide mixed with methane can prove to be economically advantageous ifthe receiving geological structure is at a sufficiently high temperatureand the use of the mixture with methane can avoid the necessity of aseparation process of methane from hydrogen sulfide.

Finally, it should be noted that the disposal system of sulfur, objectof the present invention, allows part of the hydrogen sulfide separatedfrom the associated gas to be used to fluidize the sulfur produced withthe Claus process, thus reducing the total volume of H₂S to be sent tothe Claus process itself.

Therefore, with respect to the practical embodiment of the presentinvention, the disposal process of sulfur should therefore be effectedby selecting a suitable receptive geological structure having atemperature ranging from 90° C. to 160° C., optionally mixing the sulfurwith hydrogen sulfide (preferably pure) so that the melting point of themixture is lower than or equal to that of the receiving geologicalstructure, heating the sulfur (or mixture) to a temperature which issufficiently higher than the melting point so that, during the pumpingprocess at the surface and in the well, the sulfur (or mixture) remainsin the liquid state and, finally, pumping the sulfur (or mixture) intothe reservoir through possibly insulated pipes.

1. A process for the disposal of sulfur comprising: melting sulfur toprovide molten sulfur; and injecting the molten sulfur into a geologicalstructure having a temperature ranging from 90 to 160° C., wherein thegeological structure is a matrix geological structure or a fracturedstructure.
 2. The process according to claim 1, wherein the sulfur comesfrom purification treatment of hydrocarbons of a fossil nature or from anatural gas.
 3. The process according to claim 1, wherein the moltensulfur comes directly from the Claus process.
 4. The process accordingto claim 1, wherein the sulfur comes from a surface storage site.
 5. Theprocess according to claim 1, wherein the geological structure is amatrix geological structure.
 6. The process according to claim 1,wherein the geological structure is a fractured structure.
 7. A processfor the disposal of sulfur comprising: melting sulfur to provide moltensulfur; and injecting the molten sulfur into a geological structurehaving a temperature ranging from 90 to 160° C., wherein melting sulfuris performed in the presence of hydrogen sulfide.
 8. The process ofclaim 7, wherein the hydrogen sulfide is used in a mixture with othergases which do not have the capacity of reducing the melting point ofthe sulfur.
 9. The process of claim 8, wherein the gas mixed withhydrogen sulfide is methane or carbon dioxide.
 10. The process of claim7, wherein the hydrogen sulfide is separated from natural gas.
 11. Theprocess according to claim 7, wherein the sulfur comes from purificationtreatment of hydrocarbons of a fossil nature or from a natural gas. 12.The process according to claim 7, wherein the molten sulfur comesdirectly from the Claus process.
 13. The process according to claim 12,wherein the geological structure is a naturally fractured structure. 14.The process according to claim 12, wherein the geological structure isan induced fractured structure.
 15. The process according to claim 7,wherein the sulfur comes from a surface storage site.
 16. The processaccording to claim 7, wherein the geological structure is one formingthe reservoir from which crude oil or natural gas containing sulfur areremoved.